The present invention relates to a method and an apparatus for the continuous production of expanded plastic material for forming panels and the like.
It is known that expanded polyurethane foams are typically obtained by pouring a so-called reactive, liquid mixture, constituted by the reagents, i.e., isocyanate and polyol, water, catalysts and so forth, and by any blowing agents such as hydrocarbons, hydrochlorofluorocarbons, hydrofluorocarbons, and the like.
In a few seconds, the reactive mixture starts to react, generating heat and carbon dioxide produced by the water-isocyanate reaction.
The carbon dioxide, together with the gases generated by the evaporation of any blowing agents, due to the heat generated by the chemical reactions, causes the expansion of the reactive mixture so as to generate the foam.
On the basis of this type of method, plants have been available for many years which allow the continuous production of low-density rigid foam panels for thermal insulation having, in cross-section, a rectangular shape or otherwise variously shaped; these panels can be produced in different lengths.
The foam of the lower and upper surfaces of the panels is coupled to substrates of various kinds, such as for example paper, bituminized paper, wood, metal plate and the like, which form outer claddings of said panel.
These panels are widely used, in particular, as insulators in the field of industrial building or in the field of industry or refrigeration.
Plants for manufacturing the panels generally have a series of dosage lines which dose out the components in the intended ratios to a fixed or movable mixing head which feeds the lower substrate of the cladding with the mixture produced by said lines, optionally by means of appropriately provided distribution units.
In another application, the reactive mixture can be applied by deposit on the upper substrate.
The reactive mixture enters a step of growth and polymerization and is conveyed, together with the upper and lower claddings, by appropriately provided movable conveyance and containment systems such as, for example, double conveyor belts, to a cutter for cutting the panels to the intended length.
A fundamental aspect in obtaining good-quality panels is the manner in which the reactive mixture is distributed by the mixing head onto the substrate of the cladding.
Accordingly, several systems have been proposed according to the type of panel to be produced and to the production rates; these systems use, for example, multiple fixed heads or a head which performs a reciprocating motion on a guide which is perpendicular to the panel production axis.
In the latter case, it is particularly important that the head be lightweight and compact, so as to allow easy placement of the injection head between the lower and upper substrates of the panel before entering the double containment belt.
In any event, however, containment of the weight and dimensions of the heads is a desideratum to achieve plant optimization.
In most cases, the production of thermal insulation panels requires the use of the cited blowing agents, and therefore such blowing agents are preferably added continuously at the time of production, with the advantage of being able to control their dosage according to the physical characteristics of the panel to be produced.
The addition can be performed directly in the mixing head or by premixing, in the exact amount required, by means of static or dynamic mixers, the blowing agent with one of the reactive components, for example the polyol, before said component, drawn from the storage tank, reaches the head in which final mixing occurs in order to prepare the reactive mixture to be deposited by pouring onto the substrate of the panel.
What has been described above for the blowing agent can in any case be extended to other components, catalysts and the like that are involved in the formation of the reactive mixture.
The concept already well known is that it is not convenient to perform the addition directly in the injection head, since an immediate consequence would be an increase in dimensions and weight, in addition to an increase in constructive complexity.
If high-pressure injection heads are used, problems are encountered in relation to the premixing of the various components, which is performed in the low-pressure part of the circuit, i.e., upstream of the high-pressure delivery pump.
This aspect causes considerable problems, since an inevitable seepage of the polyol high-pressure pump occurs which, exactly in the case of low-pressure premixing, can cause continuous changes in the ratio among the various components.
It is also known, in polyurethane foaming technology, that precise dosage and optimum thermostatic control of the components fed to the mixing head throughout the process are extremely important for obtaining products having optimum and constant characteristics.
For this purpose, for example, it is useful to provide, before the step of pouring the reactive mixture begins, a recirculation of the components which allows to obtain the required temperature, flow-rate and pressure conditions, thus avoiding transients, in the initial periods of the pouring process, in which these parameters are not ideal, consequently producing panels which do not have the intended quality characteristics.
In the current state of the art, therefore, the possibility to provide effective recirculation becomes an important factor in ensuring that the physical conditions of the components of the reactive mixture remain constant both during the pouring steps and during the recirculation steps.